1. Field of the Invention
The invention concerns a detection and location apparatus and method. Polarization effects are managed and used as the parameter that permits detection of an event that produces a local disturbance in optical characteristics, and determination of the location of the disturbance, to a point along the length of an optical waveguide that forms a distributed sensor.
In one embodiment, a polarization change is developed by establishing two counter-propagating light signals carried by at least one waveguide. The counter-propagating light signals are locally affected by the substantially same disturbance. The effect of the disturbance on the counter-propagating light signals is detected, with a temporal shift, after the light signals have propagated in opposite directions to a detector. The temporal shift is used to calculate the location at which the disturbance affected the light signals.
The waveguide can be an optical fiber or two or more optical fibers or plural modes in a given fiber, in each case supporting propagation of a beam in the waveguide. The opposite light signals can originate from different light sources and/or can be subdivided beams from a same source. Although propagating in opposite directions, the two light signals are affected by the disturbance in substantially the same way.
Polarization aspects of the at least two counter-propagating light signals are established using polarization sensitive beam splitter/combiner elements coupled to the counter-propagating signal channels. A phase difference between beams for each of the counter-propagating light signals is established. In the detection zone, a localized event can cause a disturbance in optical transmission properties such as the refractive index. The changes that result in the two signals are temporally resolved to determine the location of the disturbance to a point along the waveguide, at least to a tolerance.
Physical disturbances such as pressure or stress from moving masses and other events of potential security interest cause polarization altering changes in both of the counter-propagating optical signals. Such changes are detectable according to the invention with sensitivity and precision. The optical fiber waveguide medium is insensitive to electromagnetic interference, intrinsically safe, stable and reliable. However, at the scale of the wavelength of the light signals, momentary stresses and the like produce variations that are readily detectable as phase variations leading to a change in polarization states.
Although the disclosed technology can be applied to various position sensing situations, this disclosure uses the example of optical fiber based perimeter security as a non-limiting example. Inasmuch as an optical waveguide is easily placed to follow various paths, the same technique can be used to extend a detection path between arbitrary zones, to provide a two or three dimensional detection area, etc.
2. Prior Art
A security system should detect and provide information about any intrusion into a protected area or facility. An advantageous system should discreetly detect even modest physical disturbances, and report the location of the disturbance so as to permit corrective action to ensue promptly. If a security system is not visible or otherwise apparent to an intruder, it is more difficult for the intruder to proceed undetected than if elements of the security system are not concealed. There may be a deterrence benefit, however, in making it known that a facility is equipped with security devices.
Some optical sensors rely on gross effects of an intruder's presence, such as the intruder interrupting a beam that is aimed from a source to a sensor. Other sensors rely on proximity or the like. Whether the effect is gross or subtle, there is a need to know not only that a disturbance has occurred but also to know where the disturbance occurred. With one signal path, it may be possible from changes in the received signal to determine that a disturbance has occurred, but not to know where. One technique for localizing a disturbance is by determining the difference in timing between the appearance of effects of a disturbance, in two signals that are both affected by the disturbance. A relative delay in appearance of the disturbance in a signal propagating on one path versus another path, indicates a longer propagation distance from the disturbance to the detector, where the signal is detected. If there are two or more operative paths, measuring the delay can permit one to calculate an apparent location of the disturbance. This technique is described in British Patent GB 1,497,995—Ramsay, entitled “Fiber Optic Acoustic Monitoring Arrangement.”
Optical fiber has inherent advantages, such as low loss, immunity to electromagnetic interference and other characteristics, that are useful in remote sensing. Optical fiber interference sensors as in Ramsay have the additional advantages of geometric versatility (i.e., the fiber can follow almost any desired route), wide dynamic range, and high sensitivity, partly due to the very short wavelength of the electromagnetic radiation (light energy) that is carried in an optical fiber. The measurement of the delay in Ramsay and other similar detectors is the phase difference between light from a given source, received over two different paths, such as counter-propagating paths, of potentially different length. The phase difference is detected at the receiving end of both paths, by causing the light from the two fibers to interfere, i.e., to add constructively or destructively at a summing node. As the signals move in and out of phase, the intensity of the interference sum varies between a maximum and a minimum.
Example of an interference sensor is the Mach-Zehnder interferometer, which has been applied to acoustic sensing, magnetic sensing, temperature sensing, pressure sensing, structure monitoring, etc, including using optical fibers, as described in “Overview of Mach-Zehnder sensor Technology and Applications” by Anthony Dandridge and Alan D. Kersey, Fiber Optic and Laser Sensors VI, Proc. SPIE Vol. 985, pp. 34–52 (1988).
In addition to GB 1,497,995—Ramsay, cited above, the publication “Fiber Optic Distributed Sensor in Mach-Zehnder Interferometer Configuration” by Bogdan Kizlik, TCSET'2002 Lviv-Slavsko, Ukraine proposes location fixing techniques. Recent US Patents and publications including U.S. Pat. No. 6,621,947 and US2003198425 teach the possibility of a perimeter defense system based on the same principle.
These prior art teachings rely on interference techniques to produce a variation in intensity that reflects the parameter that is needed to determine a location from a difference in propagation time over two distinct signal paths. There is a problem, however, when attempting to use optical fiber waveguides and the like for location detection because polarization effects and polarization induced phase delays can defeat the ability of an interferometer to produce a robust and dependable signal.
For light waves to interfere, there must be some correspondence in polarization alignment. Two light waves that are plane polarized and orthogonally aligned cannot interfere. Over plural paths between a light source and two or more detectors, each passing a point of disturbance, the birefringence of different fibers inside an optical path can change the polarization alignment and phase characteristics of the light beams. The birefringence of an optical fiber may be small compared to its refractive index. Nevertheless, an accumulated polarization effect arises, particularly over a long distance, and can be a large effect on a wavelength scale. An interferometer-based system cannot perform consistently, and in some circumstances will not perform at all if polarization effects cause the polarization states of the intended interfering counter-propagating optical signals to vary between alignments in the signals are more or less parallel and orthogonal at different times.
Adverse effects on interfering beams due to polarization state changes in a single light path is known as polarization-induced fading. The problem is described, for example, in “Polarization-Induced Fading in Fiber-Optic Sensor Arrays” (Moshe Tur, Yuval S. Boger, and H. J. Shaw, Journal of Lightwave Technology, Vol. 13, No. 7, p1269, 1995). This publication seeks to enhance the visibility of the interference beam in a single-channel fiber based interferometer, where the light travels along a single direction.
An interferometer produces an intensity response by causing phase varying signals to add or to cancel at different phase positions (i.e., to interfere), and as a result, the effect of polarization fading and polarization induced phase shift can be quite detrimental, leading to system failure if precautions are not taken. Occasional or uncontrollable system failure is unacceptable for a system deployed for security purposes. GB 1,497,995—Ramsay (supra) and other known fiber based perimeter security systems as described, detect variations in intensity from interfering two beams and are subject to fading and phase shift with changes in polarization of beams passed through a fiber interferometer in opposite directions. This limits effectiveness of such systems.
The present invention avoids fading and phase shift problems by establishing conditions that provide a robust response notwithstanding time changing polarization transformation characteristics such as birefringence. According to an advantageous aspect, using changing polarization characteristics as the parameter that is sensed and from which location information is resolved, as opposed to using intensity changes in interfering signals, the invention not only provides a versatile and effective disturbance detection system but also solves the prior art difficulty with polarization fading and polarization induced phase shift. This makes the invention practical and effective in perimeter security systems, as well as distributed sensing for various other purposes.